<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>hover</title>
    <style>
        .a {color : #99ff66;}
        .b{
            display: none;
        }
        .a:hover .b{
            display: block;
            color : #FF0000;
        }
    </style>
</head>
<body>
<div class='a'>元素1

</div>
<div class='b'>元素2</div>
<div id='c'>元素3</div>
</body>
</html>